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A geology voyage to study fluid discharge from a rocky outcrop deep below the ocean’s surface turned up something else: A population of brooding purple octopuses. The colony is probably doomed due to the warm, low-oxygen water coming out of the rock, but those ill-fated cephalopods may be an indicator that a healthy population is hiding out nearby, a new study contends.

The octopuses reside on Dorado Outcrop, some 250 kilometers off the coast of Costa Rica and 3,000 meters deep in the sea. The outcrop is essentially a buried 23-million-year-old mountain. “Fluid is discharging from this outcrop because at some other location there’s fluid flowing from the bottom of the ocean into the earth,” notes Anne Hartwell, a marine research scientist with the University of Alaska Fairbanks. Just where that fluid is entering isn’t known, nor is the path that the water takes through the earth.

But geoscientists were curious about the fluid discharge, so in 2013 they visited the site. Images captured by the remote-controlled research vehicle Jason II revealed a wealth of brooding octopuses clinging onto the side of the outcrop. “Everyone thought they were cool, but no one really did anything about it,” Hartwell says. “There were biologists on board, but they were microbiologists.”

Hartwell wasn’t on that trip, but she was on another one a year later and visited the site in the research submarine ALVIN. “When I got to the seafloor … there was so much life,” she recalls. The octopuses weren’t even the highlight for her. “I saw sea sponges and sea stars and crabs and shrimp. And they were colorful,” she adds. “I hadn’t expected that type of macrofauna, big organisms, that deep,” she says. “And then the octopuses were there, and it was just, ‘whoa, this place is cool!’”

Those octopuses intrigued Hartwell, so she teamed up with one of the geoscientists on that original cruise, Geoffrey Wheat of the University of Alaska Fairbanks, and cephalopod expert Janet Voight of the Field Museum in Chicago. They examined hours of video and still images and other data collected on the two deep-sea missions.

A voyage to explore a rocky outcrop deep beneath the ocean’s surface revealed an ecosystem full of colorful creatures, including a colony of purple octopuses brooding on a rocky outcrop — and probably fated to quickly die.

The research team determined that the octopuses are a species of Muusoctopus, a not-well-known genus of deep-sea octopods. Hundreds of these individuals were seen during the two expeditions, including many that had cemented themselves to the outcrop and some of which appeared to be brooding eggs.

Those octopuses had attached themselves in an area of occasional fluid discharge. That fluid is 10 degrees Celsius warmer than the surrounding waters (which are a constant, and chilly, 2° C), and it contains less oxygen.

“I had initially assumed that … they really wanted to be there,” says Hartwell. It wasn’t until she took a closer look at the evidence and consulted with Voight that it became clear that the octopuses are physiologically stressed when they’re in the fluid, she says. The moms may have settled there when no fluid was flowing and are stuck when conditions change. The species should be well adapted to the cold, higher-oxygen conditions found in the deep ocean. And when they find themselves out of their normal element, the octopuses, and their young, probably don’t survive, the team reports in the May issue of Deep Sea Research I.

But it’s not all bad news. Hartwell and her colleagues think that the doomed cephalopods are an indicator of a larger population nearby. In some cases, there were just arms or part of a mantle sticking out of the rock. “That was our evidence that there were octopuses that could fit inside spaces available on this outcrop, and these spaces are notably not associated with any fluid discharge,” Hartwell says.

She is reluctant, though, to say that such a population really exists. “Because we can’t see it, we have no way of knowing whether they’re there or not,” Hartwell cautions. She hopes that scientists might one day be able to revisit the outcrop and poke around for those hidden octopuses. In the meantime, she’s working to classify the rest of that colorful community that so dazzled her deep beneath the surface of the ocean.